Infrared detectors are used in a variety of applications to provide an electrical output which is a useful measure of the incident infrared radiation. For example, quantum detectors are one type of infrared detector that are often used for night vision purposes in a variety of military, industrial and commercial applications. Quantum detectors generally operate at cryogenic temperatures and therefore require a cryogenic cooling apparatus. As a result, quantum detectors that operate at cryogenic temperatures can have a relatively complex design and generally consume significant amounts of energy.
Another type of infrared detector is a thermal detector. Thermal detectors are typically uncooled and therefore generally operate at room temperature. One type of thermal detector that has been developed and is becoming increasingly popular is a microbolometer-based, uncooled focal plane array. A focal plane array generally includes a plurality of pixel structures, each of which include a bolometer disposed upon a common substrate. Each bolometer includes a transducer element that has an electrical resistance that varies as a result of temperature changes produced by the incident infrared radiation. By detecting changes in the electrical resistance, a measure of the incident infrared radiation can be obtained. Since the design of a bolometer-based uncooled focal plane array is generally less complex than cryogenically cooled quantum detectors and since these uncooled focal plane arrays generally require significantly less energy than cryogenically cooled quantum detectors, bolometer-based uncooled focal plane arrays are being increasingly utilized.
Each pixel structure of a conventional uncooled focal plane array has a bolometer that includes an absorber element for absorbing infrared radiation and an associated transducer element having an electrical resistance that varies as its temperature correspondingly varies. Although the absorber and transducer elements can be separate layers of a multilayer structure, the absorber element and the transducer element may sometimes be the same physical element.
In operation, infrared radiation incident upon the absorber element will heat the absorber element. Since the absorber element and the transducer element are in thermal contact, the heating of the absorber element will correspondingly heat the transducer element, thereby causing the electrical resistance of the transducer element to change in a predetermined manner. By measuring the change in electrical resistance of the transducer element, such as by passing a known current through the transducer element, a measure of the incident radiation can be obtained.
In order to permit the bolometer to be responsive to changes in the incident infrared radiation, the bolometer is generally designed to minimize thermal loss to the substrate. Thus, the bolometers of conventional focal plane arrays have separated the absorber and the transducer elements from the substrate so as to substantially thermally decouple the relatively massive substrate from the pixel. In this regard, each bolometer generally includes two or more legs that support the absorber and transducer elements above the substrate. The legs can extend between the absorber and transducer elements and the substrate, or the legs can connect the absorber and transducer elements to pillars or the like that support the absorber and transducer elements above the substrate.
In order to provide thermal contact between the absorber and the transducer elements while electrically insulating the transducer element from the absorber element, the bolometer also generally includes a thermally conductive, electrically insulating layer disposed between the absorber element and the transducer element. In addition, the bolometer typically includes another insulating layer disposed on the surface of the bolometer facing the substrate which serves to structurally support the other layers and to protect the other layers during the fabrication process. See, for example, U.S. Pat. Nos. 5,286,976; 5,288,649; 5,367,167 and 6,307,194 which describe the pixel structures of conventional bolometer-based focal plane arrays, the contents of each of which are incorporated herein by reference.
In order to further improve the performance of conventional pixel structures, each bolometer can include a reflector disposed upon the surface of the substrate underlying the absorber and transducer elements. As such, infrared radiation that is incident upon the bolometer, but that passes through and is not absorbed by the absorber element, will be reflected by the reflector back towards the absorber element. At least a portion of the reflected radiation will therefore be absorbed by the absorber element during its second pass through the absorber element, thereby increasing the percentage of the incident radiation that is absorbed by the absorber element.
However, the pixel structure of conventional bolometer-based focal plane arrays, such as disclosed in U.S. Pat. No. 6,307,194, still allow a substantial percentage of reflected radiation to pass through and exit the absorber without being absorbed. Moreover, in conventional bolometer-based focal plane arrays, the absorber has a resistive absorption layer that has a typical thickness of 50 Angstroms or more using current fabrication and deposition techniques. Accordingly, the absorber in conventional bolometer-based focal plane arrays has a resistivity of 200-350 ohms per square, which limits the absorption capabilities of the conventional pixel structure.
Therefore, there is a need for an improved absorber that overcomes the problems noted above and others previously experienced for bolometers. In particular, there is a need for a bolometer that has an absorber with a high resistivity and increased absorption capabilities to effectively absorb substantially all incident radiation